1. Field of the Invention
This invention relates to new and useful improvements in geothermal wells and more particularly to geothermal expansion wellhead systems for completing geothermal wells and providing for expansion and contraction of the production casing due to thermal cycling.
2. Description of the Prior Art
In recent years, the drilling of wells for production of geothermal energy has become commercially important. In the drilling of geothermal wells, a well bore is drilled into a formation located at a sufficient depth in the earth to be at a very high temperature and having a source of water flowing into the formation. As a result, such a formation has superheated hot water and or high temperature steam available to be reduced to the surface through the geothermal well. Geothermal wells are a source of substantially renewable energy in that they tap the natural heat of the earth's core.
The drilling and completion of geothermal wells is substantially different from the drilling and completion of oil, gas and water wells. This primarily due to the fact that the drilling extends into very high temperature formations where special drilling equipment may be required. Problems are additionally encountered in the completion of geothermal wells in that the well casing which extends from the high temperature area to conduct high temperature pressurized hot water and/or steam to the surface is subject to expansion and contraction due to thermal cycling. The expansion and contraction of the well casing due to thermal cycling may result in a very substantial movement of the upper end of the production casing in response to temperature changes.
In the past, geothermal wells have been completed by use of ordinary wellheads which may move up and down with the expansion and contraction of the production casing. This necessitated the use of flexible connections to conduct the high temperature hot water and or steam to the point of utilization. The movement of the wellhead with expansion and contraction of the production casing has presented numerous problems in the production of energy from geothermal wells.
Another approach to the completion of geothermal wells, has been the use of expansion joints in the wellhead assembly. One commercially available geothermal wellhead system (no published literature or patents are known which describe this system) has an expansion joint which includes an expansion spool in which the production casing is allowed to expand or contract. An annular seal is positioned between the production spool and the exterior surface of the production casing to seal off the pressure from below. The production casing can become corroded or roughened with deposits of material during production of steam and hot water from the well and the deposits on the casing may result in undue wear of the pressure seal in the expansion spool.
There has been a substantial need for a well designed geothermal wellhead expansion system in which the seals are easily serviced and the sealing does not take place against a rough surface such as that encountered on production casing.